The present invention relates to a process for hydrogenating nitrites to primary amines over an activated Raney catalyst.
It is known that nitriles and iminonitriles can be hydrogenated in the liquid phase over Raney catalysts.
EP-A-0 382 508 describes the semicontinuous hydrogenation of polynitriles in the liquid phase over Raney cobalt catalysts in the presence of anhydrous ammonia.
U.S. Pat. No. 5,869,653 describes a continuous process for hydrogenating nitrites over Raney cobalt catalysts in the absence of ammonia, which is carried out in the presence of catalytic amounts of lithium hydroxide and water.
U.S. Pat. No. 4,895,994 discloses a Raney catalyst having a BET surface area of from 20 to 80 m2/g and a proportion of macropores of from 0.01 to 70% by volume, based on the total pore volume, which is produced by mixing the Raney alloy with a high molecular weight polymer, shaping the mixture to form a shaped body, calcining the composition firstly at from 300 to 700xc2x0 C. and subsequently at from 850 to 1200xc2x0 C. in the presence of oxygen and leaching aluminum from the calcined shaped body by treatment with 6 N NaOH at from 90 to 100xc2x0 C. The catalyst which has been activated in this way is subsequently washed repeatedly with water until the pH of the washings is  less than 9. The Raney catalyst is used, inter alia, for hydrogenating nitrites to amines.
EP-A-0 842 699 discloses a process for producing an activated, metal powder-free, macroporous fixed-bed metal catalyst of the Raney type based on an alloy of aluminum and at least one metal of transition group VIII of the Periodic Table, which comprises the steps (1) preparing a kneadable composition comprising the alloy, a shaping aid, water and a pore former, (2) shaping the kneadable composition to give a shaped body, (3) calcining the shaped body and (4) treating the calcined shaped body with an alkali metal hydroxide. After treatment of the shaped body with alkali metal hydroxide, the activated catalyst is washed with water until the pH of the washings has dropped to 7.5. The catalyst obtained in this way has a macropore content of more than 80% by volume. The catalyst is used for hydrogenating nitrites to primary amines.
Such a process is also disclosed in DE-A 44 46 907, in which polyvinyl alcohol and water or stearic acid are used as auxiliaries.
A disadvantage of the semicontinuous or continuous processes of the prior art is that they are carried out either in the presence of ammonia as solvent or in the presence of basic alkali metal compounds such as LiOH. The hydrogenation in the presence of ammonia brings with it an additional engineering outlay associated with the separation, work-up and recirculation of the ammonia. Basic alkali metal compounds are corrosive and their use therefore likewise requires additional engineering outlay.
It is an object of the present invention to provide a simple-to-carry out process for the gentle hydrogenation of nitrites, in particular to provide a process of this type which displays high selectivity in respect of the formation of primary amines and in which secondary reactions such as dissociation of the nitrites are avoided.
We have found that this object is achieved by a process for the continuous hydrogenation of nitrites to primary amines in the liquid phase over a suspended, activated Raney catalyst based on an alloy of aluminum and at least one transition metal selected from the group consisting of iron, cobalt and nickel, and, if desired, one or more further transition metals selected from the group consisting of titanium, zirconium, chromium and manganese, wherein the hydrogenation is carried out in the absence of ammonia and basic alkali metal compounds or alkaline earth metal compounds.
Catalysts which are preferably used in the process of the present invention are produced on the basis of an alloy of aluminum and a solid solution of nickel in cobalt or cobalt in nickel comprising 0.05-50% by weight of the dissolved metal and, if desired, from 0.05 to 5% by weight of the further transition metal or metals.
The catalysts are preferably activated by (a) treatment with aqueous alkali metal hydroxide solution, if desired (b) rinsing the catalyst with aqueous alkali metal hydroxide solution and, if desired, (c) rinsing the catalyst with water.
Preferred transition metals are nickel and cobalt; particular preference is given to solid solutions of nickel in cobalt or cobalt in nickel in which the dissolved metal is present in a concentration of from 0.05 to 50% by weight. To increase the activity and selectivity, the alloy may further comprise at least one additional transition metal selected from among titanium, zirconium, chromium and manganese as promoter, generally in concentrations of from 0.01 to 15% by weight, preferably from 0.05 to 5% by weight, based on the total amount of transition metals. The weight ratio of aluminum to transition metal is generally in the range from 30 to 70% by weight of aluminum and from 30 to 70% by weight of transition metal.
The aluminum alloy is produced in a manner known per se, for example as described in DE 21 59 736, whose contents relating to the production of alloys of aluminum and the specified transition metals are hereby incorporated by reference into the present application.
The calcined shaped bodies are, according to the present invention, activated using an alkali metal hydroxide, preferably lithium hydroxide, sodium hydroxide, potassium hydroxide or cesium hydroxide or a mixture thereof, particularly preferably using sodium hydroxide alone or in admixture with the abovementioned alkali metal hydroxides. In general, use is made of aqueous solutions of the alkali metal hydroxide, preferably aqueous sodium hydroxide solution, with the alkali metal hydroxide solution having a concentration of generally from 5 to 30% by weight, preferably from 15 to 25% by weight. The molar ratio of alkali metal hydroxide to aluminum is generally from about 1:1 to about 5:1, preferably from about 1.5:1 to about 3:1.
The activation temperature is usually from about 25xc2x0 C. to about 106xc2x0 C., preferably from about 45xc2x0 C. to about 90xc2x0 C.
The activation time depends essentially on the desired final aluminum content and is in the range from 1 to 10 hours, preferably from 2 to 5 hours. The activation procedure can also be carried out a number of times.
After the activation procedure, the shaped catalyst body can be rinsed with water. The shaped catalyst body is, after the activation, preferably rinsed firstly with aqueous alkali metal hydroxide solution and subsequently with water.
Preferably, rinsing is carried out using an aqueous solution of the alkali metal hydroxide having a concentration of generally from 5 to 30% by weight, preferably from 15 to 25% by weight. Particularly preferably, rinsing is carried out using the same alkali metal hydroxide used in the activation of the catalyst and the alkali metal hydroxide concentration of the rinsing solution is the same as that of the alkali metal hydroxide solution used for activation. Rinsing is preferably carried out at room temperature. Rinsing with alkali metal hydroxide solution can be carried out a number of times, preferably three times, with the total amount of the alkali metal hydroxide solution used being similar to that used in the activation. Only then is rinsing with water carried out, preferably until the pH of the washings is about 8.
The washed, activated shaped catalyst bodies are stored under water, preferably in a mixture of water and methanol.